Advantage II effectively kills 98% to 100% of the fleas on cats and dogs within 12 hours after application. One treatment prevents further flea infestations for up to 4 weeks. Reinfesting fleas are killed within 2 hours. Flea larvae are killed following contact with an Advantage II treated pet. Active ingredient is imidacloprid 9.1%. Advantage II is a once-a-month topical application for cats and kittens 8 weeks and older and dogs and puppies 7 weeks and older. Always read package labeling and directions prior to using AdvantageII.